lnd.xprv/channeldb/kvdb/etcd/stm.go

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package etcd
import (
"context"
"fmt"
"math"
"strings"
v3 "github.com/coreos/etcd/clientv3"
)
// KV stores a key/value pair.
type KV struct {
key string
val string
}
// STM is an interface for software transactional memory.
// All calls that return error will do so only if STM is manually handled and
// abort the apply closure otherwise. In both case the returned error is a
// DatabaseError.
type STM interface {
// Get returns the value for a key and inserts the key in the txn's read
// set. Returns nil if there's no matching key, or the key is empty.
Get(key string) ([]byte, error)
// Put adds a value for a key to the txn's write set.
Put(key, val string)
// Del adds a delete operation for the key to the txn's write set.
Del(key string)
// First returns the first k/v that begins with prefix or nil if there's
// no such k/v pair. If the key is found it is inserted to the txn's
// read set. Returns nil if there's no match.
First(prefix string) (*KV, error)
// Last returns the last k/v that begins with prefix or nil if there's
// no such k/v pair. If the key is found it is inserted to the txn's
// read set. Returns nil if there's no match.
Last(prefix string) (*KV, error)
// Prev returns the previous k/v before key that begins with prefix or
// nil if there's no such k/v. If the key is found it is inserted to the
// read set. Returns nil if there's no match.
Prev(prefix, key string) (*KV, error)
// Next returns the next k/v after key that begins with prefix or nil
// if there's no such k/v. If the key is found it is inserted to the
// txn's read set. Returns nil if there's no match.
Next(prefix, key string) (*KV, error)
// Seek will return k/v at key beginning with prefix. If the key doesn't
// exists Seek will return the next k/v after key beginning with prefix.
// If a matching k/v is found it is inserted to the txn's read set. Returns
// nil if there's no match.
Seek(prefix, key string) (*KV, error)
// OnCommit calls the passed callback func upon commit.
OnCommit(func())
// Commit attempts to apply the txn's changes to the server.
// Commit may return CommitError if transaction is outdated and needs retry.
Commit() error
// Rollback emties the read and write sets such that a subsequent commit
// won't alter the database.
Rollback()
}
// CommitError is used to check if there was an error
// due to stale data in the transaction.
type CommitError struct{}
// Error returns a static string for CommitError for
// debugging/logging purposes.
func (e CommitError) Error() string {
return "commit failed"
}
// DatabaseError is used to wrap errors that are not
// related to stale data in the transaction.
type DatabaseError struct {
msg string
err error
}
// Unwrap returns the wrapped error in a DatabaseError.
func (e *DatabaseError) Unwrap() error {
return e.err
}
// Error simply converts DatabaseError to a string that
// includes both the message and the wrapped error.
func (e DatabaseError) Error() string {
return fmt.Sprintf("etcd error: %v - %v", e.msg, e.err)
}
// stmGet is the result of a read operation,
// a value and the mod revision of the key/value.
type stmGet struct {
val string
rev int64
}
// readSet stores all reads done in an STM.
type readSet map[string]stmGet
// stmPut stores a value and an operation (put/delete).
type stmPut struct {
val string
op v3.Op
}
// writeSet stroes all writes done in an STM.
type writeSet map[string]stmPut
// stm implements repeatable-read software transactional memory
// over etcd.
type stm struct {
// client is an etcd client handling all RPC communications
// to the etcd instance/cluster.
client *v3.Client
// manual is set to true for manual transactions which don't
// execute in the STM run loop.
manual bool
// options stores optional settings passed by the user.
options *STMOptions
// prefetch hold prefetched key values and revisions.
prefetch readSet
// rset holds read key values and revisions.
rset readSet
// wset holds overwritten keys and their values.
wset writeSet
// getOpts are the opts used for gets.
getOpts []v3.OpOption
// revision stores the snapshot revision after first read.
revision int64
// onCommit gets called upon commit.
onCommit func()
}
// STMOptions can be used to pass optional settings
// when an STM is created.
type STMOptions struct {
// ctx holds an externally provided abort context.
ctx context.Context
}
// STMOptionFunc is a function that updates the passed STMOptions.
type STMOptionFunc func(*STMOptions)
// WithAbortContext specifies the context for permanently
// aborting the transaction.
func WithAbortContext(ctx context.Context) STMOptionFunc {
return func(so *STMOptions) {
so.ctx = ctx
}
}
// RunSTM runs the apply function by creating an STM using serializable snapshot
// isolation, passing it to the apply and handling commit errors and retries.
func RunSTM(cli *v3.Client, apply func(STM) error, so ...STMOptionFunc) error {
return runSTM(makeSTM(cli, false, so...), apply)
}
// NewSTM creates a new STM instance, using serializable snapshot isolation.
func NewSTM(cli *v3.Client, so ...STMOptionFunc) STM {
return makeSTM(cli, true, so...)
}
// makeSTM is the actual constructor of the stm. It first apply all passed
// options then creates the stm object and resets it before returning.
func makeSTM(cli *v3.Client, manual bool, so ...STMOptionFunc) *stm {
opts := &STMOptions{
ctx: cli.Ctx(),
}
// Apply all functional options.
for _, fo := range so {
fo(opts)
}
s := &stm{
client: cli,
manual: manual,
options: opts,
prefetch: make(map[string]stmGet),
}
// Reset read and write set.
s.Rollback()
return s
}
// runSTM implements the run loop of the STM, running the apply func, catching
// errors and handling commit. The loop will quit on every error except
// CommitError which is used to indicate a necessary retry.
func runSTM(s *stm, apply func(STM) error) error {
out := make(chan error, 1)
go func() {
defer func() {
// Recover DatabaseError panics so
// we can return them.
if r := recover(); r != nil {
e, ok := r.(DatabaseError)
if !ok {
// Unknown panic.
panic(r)
}
// Return the error.
out <- e.Unwrap()
}
}()
var err error
// In a loop try to apply and commit and roll back
// if the database has changed (CommitError).
for {
// Abort STM if there was an application error.
if err = apply(s); err != nil {
break
}
err = s.Commit()
// Re-apply only upon commit error
// (meaning the database was changed).
if _, ok := err.(CommitError); !ok {
// Anything that's not a CommitError
// aborts the STM run loop.
break
}
// Rollback before trying to re-apply.
s.Rollback()
}
// Return the error to the caller.
out <- err
}()
return <-out
}
// add inserts a txn response to the read set. This is useful when the txn
// fails due to conflict where the txn response can be used to prefetch
// key/values.
func (rs readSet) add(txnResp *v3.TxnResponse) {
for _, resp := range txnResp.Responses {
getResp := (*v3.GetResponse)(resp.GetResponseRange())
for _, kv := range getResp.Kvs {
rs[string(kv.Key)] = stmGet{
val: string(kv.Value),
rev: kv.ModRevision,
}
}
}
}
// gets is a helper to create an op slice for transaction
// construction.
func (rs readSet) gets() []v3.Op {
ops := make([]v3.Op, 0, len(rs))
for k := range rs {
ops = append(ops, v3.OpGet(k))
}
return ops
}
// cmps returns a cmp list testing values in read set didn't change.
func (rs readSet) cmps() []v3.Cmp {
cmps := make([]v3.Cmp, 0, len(rs))
for key, getValue := range rs {
cmps = append(cmps, v3.Compare(v3.ModRevision(key), "=", getValue.rev))
}
return cmps
}
// cmps returns a cmp list testing no writes have happened past rev.
func (ws writeSet) cmps(rev int64) []v3.Cmp {
cmps := make([]v3.Cmp, 0, len(ws))
for key := range ws {
cmps = append(cmps, v3.Compare(v3.ModRevision(key), "<", rev))
}
return cmps
}
// puts is the list of ops for all pending writes.
func (ws writeSet) puts() []v3.Op {
puts := make([]v3.Op, 0, len(ws))
for _, v := range ws {
puts = append(puts, v.op)
}
return puts
}
// fetch is a helper to fetch key/value given options. If a value is returned
// then fetch will try to fix the STM's snapshot revision (if not already set).
// We'll also cache the returned key/value in the read set.
func (s *stm) fetch(key string, opts ...v3.OpOption) ([]KV, error) {
resp, err := s.client.Get(
s.options.ctx, key, append(opts, s.getOpts...)...,
)
if err != nil {
dbErr := DatabaseError{
msg: "stm.fetch() failed",
err: err,
}
// Do not panic when executing a manual transaction.
if s.manual {
return nil, dbErr
}
// Panic when executing inside the STM runloop.
panic(dbErr)
}
// Set revison and serializable options upon first fetch
// for any subsequent fetches.
if s.getOpts == nil {
s.revision = resp.Header.Revision
s.getOpts = []v3.OpOption{
v3.WithRev(s.revision),
v3.WithSerializable(),
}
}
var result []KV
// Fill the read set with key/values returned.
for _, kv := range resp.Kvs {
// Remove from prefetch.
key := string(kv.Key)
val := string(kv.Value)
delete(s.prefetch, key)
// Add to read set.
s.rset[key] = stmGet{
val: val,
rev: kv.ModRevision,
}
result = append(result, KV{key, val})
}
return result, nil
}
// Get returns the value for key. If there's no such
// key/value in the database or the passed key is empty
// Get will return nil.
func (s *stm) Get(key string) ([]byte, error) {
if key == "" {
return nil, nil
}
// Return freshly written value if present.
if put, ok := s.wset[key]; ok {
if put.op.IsDelete() {
return nil, nil
}
return []byte(put.val), nil
}
// Populate read set if key is present in
// the prefetch set.
if getValue, ok := s.prefetch[key]; ok {
delete(s.prefetch, key)
s.rset[key] = getValue
}
// Return value if alread in read set.
if getVal, ok := s.rset[key]; ok {
return []byte(getVal.val), nil
}
// Fetch and return value.
kvs, err := s.fetch(key)
if err != nil {
return nil, err
}
if len(kvs) > 0 {
return []byte(kvs[0].val), nil
}
// Return empty result if key not in DB.
return nil, nil
}
// First returns the first key/value matching prefix. If there's no key starting
// with prefix, Last will return nil.
func (s *stm) First(prefix string) (*KV, error) {
return s.next(prefix, prefix, true)
}
// Last returns the last key/value with prefix. If there's no key starting with
// prefix, Last will return nil.
func (s *stm) Last(prefix string) (*KV, error) {
// As we don't know the full range, fetch the last
// key/value with this prefix first.
resp, err := s.fetch(prefix, v3.WithLastKey()...)
if err != nil {
return nil, err
}
var (
kv KV
found bool
)
if len(resp) > 0 {
kv = resp[0]
found = true
}
// Now make sure there's nothing in the write set
// that is a better match, meaning it has the same
// prefix but is greater or equal than the current
// best candidate. Note that this is not efficient
// when the write set is large!
for k, put := range s.wset {
if put.op.IsDelete() {
continue
}
if strings.HasPrefix(k, prefix) && k >= kv.key {
kv.key = k
kv.val = put.val
found = true
}
}
if found {
return &kv, nil
}
return nil, nil
}
// Prev returns the prior key/value before key (with prefix). If there's no such
// key Next will return nil.
func (s *stm) Prev(prefix, startKey string) (*KV, error) {
var result KV
fetchKey := startKey
matchFound := false
for {
// Ask etcd to retrieve one key that is a
// match in descending order from the passed key.
opts := []v3.OpOption{
v3.WithRange(fetchKey),
v3.WithSort(v3.SortByKey, v3.SortDescend),
v3.WithLimit(1),
}
kvs, err := s.fetch(prefix, opts...)
if err != nil {
return nil, err
}
if len(kvs) == 0 {
break
}
kv := &kvs[0]
// WithRange and WithPrefix can't be used
// together, so check prefix here. If the
// returned key no longer has the prefix,
// then break out.
if !strings.HasPrefix(kv.key, prefix) {
break
}
// Fetch the prior key if this is deleted.
if put, ok := s.wset[kv.key]; ok && put.op.IsDelete() {
fetchKey = kv.key
continue
}
result = *kv
matchFound = true
break
}
// Closre holding all checks to find a possibly
// better match.
matches := func(key string) bool {
if !strings.HasPrefix(key, prefix) {
return false
}
if !matchFound {
return key < startKey
}
// matchFound == true
return result.key <= key && key < startKey
}
// Now go trough the write set and check
// if there's an even better match.
for k, put := range s.wset {
if !put.op.IsDelete() && matches(k) {
result.key = k
result.val = put.val
matchFound = true
}
}
if !matchFound {
return nil, nil
}
return &result, nil
}
// Next returns the next key/value after key (with prefix). If there's no such
// key Next will return nil.
func (s *stm) Next(prefix string, key string) (*KV, error) {
return s.next(prefix, key, false)
}
// Seek "seeks" to the key (with prefix). If the key doesn't exists it'll get
// the next key with the same prefix. If no key fills this criteria, Seek will
// return nil.
func (s *stm) Seek(prefix, key string) (*KV, error) {
return s.next(prefix, key, true)
}
// next will try to retrieve the next match that has prefix and starts with the
// passed startKey. If includeStartKey is set to true, it'll return the value
// of startKey (essentially implementing seek).
func (s *stm) next(prefix, startKey string, includeStartKey bool) (*KV, error) {
var result KV
fetchKey := startKey
firstFetch := true
matchFound := false
for {
// Ask etcd to retrieve one key that is a
// match in ascending order from the passed key.
opts := []v3.OpOption{
v3.WithFromKey(),
v3.WithSort(v3.SortByKey, v3.SortAscend),
v3.WithLimit(1),
}
// By default we include the start key too
// if it is a full match.
if includeStartKey && firstFetch {
firstFetch = false
} else {
// If we'd like to retrieve the first key
// after the start key.
fetchKey += "\x00"
}
kvs, err := s.fetch(fetchKey, opts...)
if err != nil {
return nil, err
}
if len(kvs) == 0 {
break
}
kv := &kvs[0]
// WithRange and WithPrefix can't be used
// together, so check prefix here. If the
// returned key no longer has the prefix,
// then break the fetch loop.
if !strings.HasPrefix(kv.key, prefix) {
break
}
// Move on to fetch starting with the next
// key if this one is marked deleted.
if put, ok := s.wset[kv.key]; ok && put.op.IsDelete() {
fetchKey = kv.key
continue
}
result = *kv
matchFound = true
break
}
// Closure holding all checks to find a possibly
// better match.
matches := func(k string) bool {
if !strings.HasPrefix(k, prefix) {
return false
}
if includeStartKey && !matchFound {
return startKey <= k
}
if !includeStartKey && !matchFound {
return startKey < k
}
if includeStartKey && matchFound {
return startKey <= k && k <= result.key
}
// !includeStartKey && matchFound.
return startKey < k && k <= result.key
}
// Now go trough the write set and check
// if there's an even better match.
for k, put := range s.wset {
if !put.op.IsDelete() && matches(k) {
result.key = k
result.val = put.val
matchFound = true
}
}
if !matchFound {
return nil, nil
}
return &result, nil
}
// Put sets the value of the passed key. The actual put will happen upon commit.
func (s *stm) Put(key, val string) {
s.wset[key] = stmPut{
val: val,
op: v3.OpPut(key, val),
}
}
// Del marks a key as deleted. The actual delete will happen upon commit.
func (s *stm) Del(key string) {
s.wset[key] = stmPut{
val: "",
op: v3.OpDelete(key),
}
}
// OnCommit sets the callback that is called upon committing the STM
// transaction.
func (s *stm) OnCommit(cb func()) {
s.onCommit = cb
}
// Commit builds the final transaction and tries to execute it. If commit fails
// because the keys have changed return a CommitError, otherwise return a
// DatabaseError.
func (s *stm) Commit() error {
// Create the compare set.
cmps := append(s.rset.cmps(), s.wset.cmps(s.revision+1)...)
// Create a transaction with the optional abort context.
txn := s.client.Txn(s.options.ctx)
// If the compare set holds, try executing the puts.
txn = txn.If(cmps...)
txn = txn.Then(s.wset.puts()...)
// Prefetch keys in case of conflict to save
// a round trip to etcd.
txn = txn.Else(s.rset.gets()...)
txnresp, err := txn.Commit()
if err != nil {
return DatabaseError{
msg: "stm.Commit() failed",
err: err,
}
}
// Call the commit callback if the transaction
// was successful.
if txnresp.Succeeded {
if s.onCommit != nil {
s.onCommit()
}
return nil
}
// Load prefetch before if commit failed.
s.rset.add(txnresp)
s.prefetch = s.rset
// Return CommitError indicating that the transaction
// can be retried.
return CommitError{}
}
// Rollback resets the STM. This is useful for uncommitted transaction rollback
// and also used in the STM main loop to reset state if commit fails.
func (s *stm) Rollback() {
s.rset = make(map[string]stmGet)
s.wset = make(map[string]stmPut)
s.getOpts = nil
s.revision = math.MaxInt64 - 1
}